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Growth and Survival of Eschericia coli O157: H7 during the Manufacturing of Ergo and Ayib, Ethiopian Traditional Fermented Milk Products

Zelalem Yilma, Gérard Loiseau and Bernard Faye

The behavior of Escherichia coli O157:H7 was evaluated during the manufacturing of Ergo (Ethiopian naturally fermented milk) and Ayib (Ethiopian cottage cheese) using a challenge test. Four E. coli O157:H7 initial inoculum levels (high: ~3x105; medium: ~3x104; low: ~3x103; and very low: ~3x102 cfu mL-1) were used together with a control milk with no lactic acid bacteria (LAB). Ayib was made by cooking defatted fermented milk after 72 h of fermentation at three cooking temperatures (50, 60 and 70°C). Samples were drawn at 0, 8, 24, 32, 48, 56 and 72 h of fermentation and at 0, 20, 40 and 60 min of cooking defatted fermented milk for bacterial enumeration and pH measurements. E. coli O157:H7 population decreased only by 1.2, 0.3 and 0.5 log cfu mL-1 after 72 h of fermentation at ambient temperature from initial counts of 5.7, 4.3 and 3.3 log cfu mL-1, respectively. Counts increased from 2.6 and 3.0 log cfu mL-1 at the start of fermentation to 2.8 and 4.2 log cfu mL-1 at the end of fermentation when co-inoculated with LAB and in control milk with no LAB, respectively. E. coli O157:H7 survived up to 60 min of cooking defatted fermented milk during Ayib-making at 50 and 60°C with counts of 2.3-2.9 and 0.8-1.5, respectively. Complete inhibition of E. coli O157:H7 was achieved between 20 and 40 min of cooking at 70°C. The direct consumption of Ergo can represent an important source of E. coli O157:H7 infections. At the inoculum levels considered in the present study, the use of cooking temperature of 70°C for at least 40 min during Ayib-making is necessary to inactivate E. coli O157:H7.